Gas expansion motor

ABSTRACT

A gas-operated motor that comprises a divergent nozzle helically wrapped around a rotary support shaft to convey gas from a high pressure inlet chamber to a low pressure outlet chamber. The gas is progressively depressurized during its flow along the nozzle passage; pressure energy is converted to a turning force on the shaft. The structure represents a relatively simple, low cost mechanism for converting gas pressure energy into high speed rotary motion.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without payment to meof any royalty thereon.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to a motor that is operated by controlledexpansion of a pressurized gas. The motor includes a rotor that has ahelical screw configuration for conveying pressurized gas from an inletchamber to an outlet chamber. The helical passage defined by the screwhas a progressively increasing cross section, measured from the inlet tothe outlet, whereby the gas expands as it travels along the passage.Expansion of the gas produces rotary motion of the rotor.

A principal aim of the invention is to provide a gas expansion motorthat has a relatively small diameter, thus reducing the need for precisedynamic balancing of the rotor. Another aim of the invention is toprovide a gas expansion motor wherein the rate of gas expansion iscontrolled by the rotor passage configuration. A further aim of theinvention is to provide a motor wherein the gas can expand from arelatively high pressure to a relatively low pressure (near atmospheric)in a single turbine stage. An important object of my invention is toprovide a gas expansion motor which operates with a minimum ofturbulence and pressure loss. A general object of the invention is toprovide a gas expansion motor that can be manufactured at relatively lowcost.

THE DRAWINGS

FIGS. 1 through 5 are longitudinal sectional views through fivedifferent embodiments of my invention.

FIG. 6 is a fragmentary sectional view through a rotor element usable inthe FIG. 2 embodiment.

FIG. 7 diagrammatically illustrates a vehicle power plant adapted to usemy improved motor.

As shown in FIG. 1, my invention comprises a gas expansion motor 10 thatincludes a housing 12 defining an inlet 14 for receiving pressurized gasfrom a non-illustrated pressure source, and an outlet 16 for dischargingspent gas in a depressurized state. A rotor 18 is disposed in housing 12for rotation around its geommetrical axis 20. The motor comprises acentral helical screw-like member 22, and two shaft sections 24 and 26extending from the ends of member 22. Anti-friction bearings 28 and 30engage the shaft sections to support the rotor for high speed rotationaround axis 20. Shaft 26 extends beyond the motor housing for connectionto a non-illustrated power-using device, such as an automotivetransmission, step-down gear box, electric generator, pump, etc.

The gas pressure at inlet 14 is appreciably higher than at outlet 16.Helical passage 32 defined by the helical wall 34 has a graduallyincreasing cross section for enabling the gas to gradually depressurizeand expand as it proceeds through the passage. The change in passagecross section is achieved by tapering the central shaft area 36 in aleft-to-right direction. Expansion of the gas resulting from progressiveincrease in passage area produces rotation of rotor 22 in a directionopposite to the direction taken by the gas; e.g. if the gas motion iscounterclockwise the rotor motion will be clockwise.

The length of helical passage 32 is preselected to maximize the gasexpansion taking place in the passage. Passage design follows designpractices used for exit nozzles of rocket engines operating just belowthe sonic flow range. Passage 32 may be visualized as being similar to alarge ratio expansion nozzle wrapped in helical fashion around a supportshaft. The linear thrust that would be achieved in a rocket engine isinstead realized as turning torque on rotor 22. I contemplate that inpractice a relatively long helical passage 32 will be used (at leastfour passage convolutions) to achieve large expansion ratios, as high asfifty. The term expansion ratio is here used to mean the ratio of thepassage transverse cross section at the right end of the rotor comparedto the passage transverse cross section at the left end of the rotor.Rate of change of the passage cross section is selected to minimizefluid separation (or overexpansion) within the passage.

FIG. 1 represents the basic concept. A preferred embodiment of theinvention is shown in FIG. 2, wherein the rotor includes an annularshroud 40 suitably attached to the outer edge areas of the helical wall34. Pressurized gas within inlet chamber 14 is admitted to helicalpassage 32 via a port 39 in shroud 40; the housing is constructed sothat port 39 communicates with inlet 14 in any rotated position of therotor. One advantage of the shrouded rotor, compared to the FIG. 1unshrouded rotor, is that the gas is effective for rotor turningpurposes immediately as it passes through port 39; in the FIG. 1 versionthe gas is believed to be fully effective only after it is confined byhousing wall surface 41, i.e, after it passes rightwardly beyond housingshoulder 43. The FIG. 2 rotor can be somewhat shorter than the FIG. 1rotor for a given gas expansion and rotor turn force.

A principal advantage of shroud 40 is that it prevents the mechanicalpower loss that might otherwise occur due to fluid leakage across theperipheral edge of the helical wall (FIG. 1). Any leakage acrossperipheral edge 35 of wall 34 in FIG. 1 results in a power loss, whereasa corresponding leakage across the labyrinth seal 37 in FIG. 2 has noeffect on the mechanical power output. Of course, in both cases theleakage will increase gas consumption.

The FIG. 3 structure is generally similar to the FIG. 2 structure.However, in the FIG. 3 embodiment helical wall 34 has a graduallydecreasing transverse cross section, measured in a left-to-rightdirection. Therefore the helical passage 32 experiences a cross sectionincrease in two different directions, i.e., radially and axially. TheFIG. 3 structure achieves a greater rate of passage 32 cross-sectionincrease than the FIG. 2 structure for a given rotor size. Actualtesting may prove the FIG. 3 version to be the most practical approachto most frequency encountered situations. FIG. 3 also illustratesbearing features that may prove desirable to meet special circumstances,especially high rotational speed requirements.

The left bearing in the FIG. 3 structure is equipped with a seal plate50 having a spring 52 for biasing the plate against a shoulder on therotor shaft. The seal element takes the form of a carbon ring 54. Therightmost bearing is an air bearing having an automatic alignmentfeature. Pressurized filtered air flows from chamber 55 through an axialpassage 56 to a chamber 58 within a bearing element 60; a filter element57 is indicated at the right end of passage 56 to insure performance ofthe air bearing. Branch passages direct the pressurized air from chamber58 into annular spaces exposed to radial bearing suface 62 and thrustbearing surface 64. The outer surface 66 of element 60 slidably seatsagainst spherical housing surface 69 to enable the bearing to readilyadjust to the exact attitude of shaft 26. A spring retainer means 68keeps element 60 within its spherical seat 69. Use of air bearings isconsidered desirable for very high rotor speed situations, e.g. above100,000 r.p.m..

The FIG. 4 embodiment is similar to the FIG. 2 embodiment except thatthe shroud has a frust-conical configuration; the rotor diameterprogressively increases from the inlet end of the rotor to the outletend. With such an arrangement the helical gas passage experiences arelatively great change in cross section, measured along the length ofthe passage. The housing internal surface may be stepped to provide amore tortuous path for fluid otherwise tending to leak through the sealelements 37 carried by the shroud wall.

FIG. 5 illustrates a form of the invention designed to include an axialgas inlet. Pressurized gas flows through axial inlet 14a into a chamber71 formed in the nose of the rotor. A turning vane 70 within chamber 71causes the gas to be rotated out of the plane of the paper and thenceinto the helical passage 32 on the rotor periphery. After its travelthrough passage 32 the now-depressurized gas exits through outlet 16.Shroud 40 includes an annular shaft section 24a whose internal surfaceengages seal elements 37a carried by a fixed annular wall 72.

The FIG. 5 structure includes air bearings for the shaft sections 24aand 26. Pressurized air flows from inlet 14a through a branch passage74; part of the passage 74 air is supplied to an annular chamber 75surrounding shaft section 24a. Part of the passage 74 air is supplied toan annular chamber 76 formed in the spherical surface of shaft-supportmember 77. A system of passages in member 77 directs the air into threeannular chambers communicating with radial bearing surface 78 and thrustbearing surfaces 80 and 82. The FIG. 5 rotor includes the tapering shaftfeature of FIGS. 1 through 4, the shroud feature of FIGS. 2 through 4,the helical wall taper feature of FIG. 3, and the shroud frusto-conicalconfiguration of FIG. 4.

FIG. 6 illustrates a rotor construction that may be used as a substitutefor the rotor shown in FIG. 2. The principal feature of interestrelative to FIG. 6 is the fact that the rotor is built to define twohelical passages 32a and 32b instead of one helical passage. Annularshroud 40 is provided with two intake ports 39a and 39b for admittingpressurized gas to respective ones of passages 32a and 32b. The onlyperceived advantage of using two (or more) helical passages is apossible capability for complete filling of each passage (large gulpfactor) at very high rotational speeds, e.g. above 200,000 r.p.m.

I believe the gas expansion motor designs shown in FIGS. 1 through 6 areusable in varying sizes and pressure ranges to suit different situationswhere a relatively small diameter structure is desired. Illustratively,FIG. 7 shows a vehicle propulsion system wherein my improved motor 10 isdisposed to drive an axial flow compressor 84 through a step-down gearbox 86. The compressed gas is passed through the cool side of an annularregenerator (heat exchanger) 88. The heated gas combusts diesel fuel ina can-type combustor 90. Highly pressurized combustion products arepassed into passage 92, thence into motor 10 for powering the rotor (notshown in FIG. 7). Low pressure gas is exhausted from motor 10 throughthe hot side of the regenerator, and thence to the atmosphere. Motor 10is a relatively small diameter, high speed device suitable for meetingvarious situations where small size is important, as in the FIG. 7vehicle propulsion environment.

The screw-like rotor design is believed to be advantageous in that thegas expansion rate is controlled by a single continuous helical passagespanning at least four helical convolutions. Flow losses (turbulence orseparation) are believed to be relatively low with this design. Anotherpossible advantage is low manufacturing cost; it is believed that thesingle rotor design can be manufactured more cheaply than conventionalmulti-stage axial flow turbine designs. Since my rotor has a relativelysmall diameter it can operate at very high rotational speeds, e.g. above200,000 r.p.m., without the centrifugal explosive failure that can occurwith conventional large diameter axial flow designs.

I known of no prior art design corresponding to my present invention.U.S. Pat. No. 1,136,957 issued in the name of C. Hettinger shows ascrew-like rotor device, which the patentee indicates can achieve agas-compressing function. I do not believe the Hettinger design can infact achieve a gas-compressing action because the volume within thescrew chamber remains the same throughout progress of the screwrotation. Various patents, such as U.S. Pat. No. 3,771,900 to Baehr,show screw-like rotors used as pumping devices. I know of no prior artdevices that approximate my design, i.e. a divergent nozzle-like passagestructure helically wrapped around a support shaft to perform a motorfunction.

I wish it to be understood that I do not desire to be limited to theexact details of construction shown and described for obviousmodifications will occur to a person skilled in the art.

I claim:
 1. A pressurized gas-expansion motor comprising a housing;spaced bearings carried by the housing and operable to define arotational axis; a pressure-operated rotor comprising a centralelongated shaft and two shaft end sections mounted in the spacedbearings for enabling the rotor on the aforementioned rotational axis;said rotor further comprising an outwardly-projecting helical wallextending circumferentially around and along the central elongated shaftfor at least four helix convolutions, and an elongated shroud carried onthe helical wall in close adjacency to an inner annular surface of thehousing; seal means preventing pressurized gas flow through the annularclearance space between the shroud and housing inner surface; saidhousing having an inlet opening near one end of the annular shroud forreceiving pressurized gas, and an outlet opening near the other end ofthe annular shroud for discharging depressurized gas; the elongatedcentral shaft, helical wall and annular shroud defining an elongatedhelical gas passage extending circumferentially around and along thecentral shaft; said shroud having a gas intake port at its inlet endcontinuously communicating the gas passage with the housing inletopening; said intake port extending only a limited distance around theshroud circumference; the defined helical gas passage having aprogressively increasing cross-sectional area, measured from the inletend of the shroud to the outlet end, whereby the gas undergoes acontinual expansion and depressurization action while it is movingthrough the gas passage; the length of the helical gas pressure beingrelated to the initial pressure of the gas received through the housinginlet opening, such that the gas exiting through the housing inletopening is at a relatively low pressure near atmospheric pressure. 2.The motor of claim 1 wherein the annular shroud is a hollow cylinder. 3.The motor of claim 1 wherein the annular shroud is frusto-conical.